There is a notion among eastern fly fishermen that river trout of the West are too easy to catch for true sport. &quot;Fat, dumb and happy,&quot; they snort. Nevertheless, the best fly fishing in America is in progress right now in a magic 100-mile circle of the Yellowstone National Park area. There, in lovely rivers like the Snake (above), feeding trout may dimple the water like rain, but an eastern expert must bring his skill along to catch them